Optical transmission may be used as a means for communication between separate integrated circuit chips (inter-chip connections) and within components on the same chip (intra-chip connections). In chip-to-chip communication via optical interconnects, each chip on the circuit board is interfaced with a transmitter-receiver optoelectronic chip, and the two optoelectronic chips are connected via a planar dielectric waveguide. Likewise, optical waveguides may be used to connect components within a chip, such as between an integrated optical source and a detector. An integrated optical waveguide is an optical path formed on a semiconductor, e.g., silicon substrate, using lithographic processing. The waveguide can be made of an inorganic crystal or semiconductor material having a higher index of refraction than the chip substrate to guide optical signals from an input optical fiber to an output optical fiber or other optical circuitry on the chip.
Light can be transmitted through an optical waveguide in one, two or many modes. Each mode travels along the axis of the waveguide with a distinct propagation constant and group velocity. A mode is described as approximately the sum of the multiple reflections of a Transverse ElectroMagnetic (TEM) wave reflecting within the core in the direction of an optical ray for all angles greater than the critical angle of total reflection, where the condition of total reflection is:
                    θ        ≥                  θ          c                    =                        sin                      -            1                          ⁡                  (                                    η              2                                      η              1                                )                      ,                  ⁢    where                      θ        c            =              critical        ⁢                                  ⁢        angle              ,                  ⁢                  η        2            =              index        ⁢                                  ⁢        of        ⁢                                  ⁢        cladding              ,                  ⁢    and              η      1        =          index      ⁢                          ⁢              core        .            When the core diameter of an optical fiber is small, only a single mode is supported and the fiber is said to be a single-mode fiber. Alignment of a single-mode fiber to an integrated optical waveguide (and vice versa) is one of the most expensive and time-consuming manufacturing processes in the packaging of semiconductor photonics. Moreover, the large difference in dimensions between a single-mode fiber (e.g., 5-9 μm diameter core) and the cross section of a waveguide on a chip (e.g., 2 μm to less than 200 nm) causes high insertion losses and packaging costs.
FIGS. 1A and 1B respectively show a perspective view and a partial cross-sectional view of a grating based vertical coupler system as described in Frederik Van Laere, et al., “Compact and Highly Efficient Grating Couplers Between Optical Fiber and Nanophotonic Waveguides,” JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 25, No. 1, January 2007. The grating based vertical coupler can be used for out-of-plane coupling between a single-mode fiber 110 and a waveguide 170 of a photonic-integrated-chip 120. As shown in FIG. 1A, the grating based vertical coupler includes a grating 100, an adiabatic taper 130 and a photonic waveguide 140 (discussed below). Referring to FIG. 1B, the waveguide 170 can be a silicon-on-insulator (SOI) waveguide made of a 220 nm thick silicon core on top of a buried oxide layer 180 on a silicon substrate 190. The grating 100 is etched into the waveguide 170 with a plurality of grating grooves 101 (e.g., twenty (20) grooves), which are invariant in the x direction. A refractive index-matching material 160 is bonded to the waveguide 170. The index-matching material 160 is not shown in FIG. 1A in order to show the optical components beneath the index-matching material 160. The refractive index of the index-matching material 160 is 1.46 to match the refractive index of the cladding of the fiber 110. The end facet of the fiber 110 is positioned close to the grating 100. The fiber 110 is slightly tilted at an angle θ of about 8 degrees to avoid reflection at the grating 100. The waveguide 170 has a finite width of about 10 μm, whereas the photonic waveguide 140 is about 0.56 μm wide. The adiabatic taper 130 is used to couple the broad waveguide 170 and the narrow photonic waveguide 140, which sends the beam to an integrated chip 150 or other optical circuitry. The grating based vertical coupler of FIGS. 1A and 1B requires the precise positioning of the fiber 110 to the grating 100 that is etched in the waveguide 170. During field operation, temperature variations, vibrations, and other environmental perturbations can cause post-bonding shifting that can adversely affect the alignment of the fiber 110 with the grating 100, resulting in insertion loss.
Other approaches for coupling an optical fiber to a chip waveguide include using microelectromechanical systems (MEMS) to align optical fibers to chips. These techniques have an added cost of requiring active MEMS alignment components and the added difficulty in fabricating and packaging the system. Accordingly, there is a need for a simplified device and system for actively aligning an optical fiber to an optical waveguide integrated on a photonics chip.